Cyclic delay diversity in a wireless system

ABSTRACT

A system includes a first transmitter, a second transmitter, and a legacy receiver. The first transmitter transmits information via a first channel to the legacy receiver. The second transmitter transmits a time-shifted version of the information via a second channel to the legacy receiver. The legacy receiver combines the information received via the first channel and the time-shifted information received via the second channel to provide combined information. The legacy receiver processes the combined information as though it is received via a single channel.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication No. 60/731,490, filed Oct. 31, 2005, which is incorporatedherein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to wireless systems, and morespecifically to techniques for providing diversity in a wireless system.

2. Background

Diversity is the use of multiple versions of a signal in a system.Diversity typically improves the performance of the system becauseanother version of the signal is available if a first version encountersa problem. Multiple versions of the signal can be provided by hardwareand/or software, for example. Digital signal processing (DSP) techniquesare often employed to provide the multiple versions. Wirelesscommunication systems in particular can employ and take advantage ofvarious types of diversity. Some examples are temporal diversity,whereby the system utilizes different copies of the signal in time, andfrequency diversity, whereby the system utilizes different copies of thesignal in frequency. For a wireless communication system that employsmultiple antennas, spatial diversity can also be utilized by the system,whereby different copies of the signal are present on each antenna.

Conventional wireless systems, such as those based on the Institute ofElectrical and Electronics Engineers (IEEE) 802.11 standard (hereinafterreferred to as “legacy wireless systems”), either are not designed tohave spatial diversity or can only use spatial diversity in a limitedmanner. A legacy wireless system, for example, includes a singletransmitter and a single receiver. The transmitter encodes data beforetransmitting the data, and the receiver decodes the encoded data forfurther processing. If the receiver has only a single antenna then itcannot benefit from spatial diversity. If it has multiple receiveantennas, it can employ some suboptimal algorithm to choose the antennahaving the highest receive power, e.g., in order to enhance the receivedsignal strength. Similarly, if the transmitter has only one antenna itcannot use spatial diversity. If it has multiple transmit antennas itcan employ some suboptimal algorithm that chooses one of the antennasbased, e.g., on the result of previous receptions on that antenna

Some modern wireless systems include multiple transmitters to improvethe transmission rate, the range, and/or the reliability of the wirelesssystem. For instance, a proposed IEEE wireless local area network (WLAN)standard, IEEE 802.11n, allows a transmission rate of up to 130 Mbps in20 MHz bandwidth by utilizing two transmitters. The proposed standard atleast doubles the transmission rates achievable using other WLANstandards. For example, IEEE 802.11a and IEEE 802.11g each support atransmission rate of up to 54 Mbps.

Example wireless systems having multiple transmitters include multipleinput, single output (MISO) systems and multiple input, multiple output(MIMO) systems. In a MISO system, multiple transmitters transmit data toa single receiver. In a MIMO system, multiple transmitters transmit datato multiple receivers.

In a conventional MISO or MIMO system, different transmitters transmitdifferent data. If two or more transmitters in a conventional MISO orMIMO system transmit the same data, then the energy transmitted by eachtransmitter cancels the energy transmitted by the other transmitter(s)at locations that are based on the distance between the respectivetransmitters.

What is needed is a method, system, and/or computer program product thataddresses one or more of the aforementioned shortcomings of conventionalwireless systems having multiple transmitters.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a method and system to provide or enhancediversity in a system. In particular, an embodiment of the presentinvention provides cyclic delay diversity in a wireless system, whereina plurality of transmitters transmit information such that a legacyreceiver is capable of processing the information received from theplurality of transmitters.

In an embodiment, a system includes a first transmitter, a secondtransmitter, and a legacy receiver. The first transmitter transmitsfirst information via a first channel. The second transmitter transmitstime-shifted first information via a second channel. The legacy receiveris capable of processing the first information and the time-shiftedfirst information as though the first information and the time-shiftedfirst information are received via a single channel. For instance, thelegacy receiver can treat the first channel and the second channel as acombined, single channel.

According to an embodiment, the first transmitter transmits the firstinformation and the second transmitter transmits the time-shifted firstinformation in accordance with a standard, such as an Institute ofElectrical and Electronics Engineers (IEEE) 802.11a standard, an IEEE802.11b standard, an IEEE 802.11g standard, or an IEEE 802.11n standard.

In another embodiment, the legacy receiver determines/estimates thesingle channel based on second information received from the firsttransmitter and time-shifted second information received from the secondtransmitter. The first information can be a data portion of anorthogonal frequency division multiplexing (OFDM) frame, and the secondinformation can be a preamble portion of the OFDM frame. The legacyreceiver may utilize an error minimization algorithm to determine thesingle channel.

In yet another embodiment, the second transmitter reduces the delayassociated with the time-shifted first or second information until thelegacy receiver is capable of determining/estimating the single channel.The second transmitter can transmit the time-shifted first or secondusing a predetermined maximum delay. If legacy receiver is not capableof determining/estimating the channel, then the second transmitter canreduce the delay and re-transmit the time-shifted first or secondinformation using the reduced delay. The second transmitter can continueto reduce the delay and re-transmit the tine-shifted first or secondinformation so long as the legacy receiver is not capable ofdetermining/estimating the channel.

The delay associated with the time-shifted first or second informationmay be programmable. The system can include a memory to store the delayassociated with the time-shifted first or second information.

Further features and advantages of the invention, as well as thestructure and operation of various embodiments of the invention, aredescribed in detail below with reference to the accompanying drawings.It is noted that the invention is not limited to the specificembodiments described herein. Such embodiments are presented herein forillustrative purposes only. Additional embodiments will be apparent topersons skilled in the relevant art(s) based on the teachings containedherein.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form partof the specification, illustrate embodiments of the present inventionand, together with the description, further serve to explain theprinciples of the invention and to enable a person skilled in thepertinent art(s) to make and use the invention. In the drawings, likereference numbers indicate identical or functionally similar elements.Additionally, the leftmost digit(s) of a reference number identifies thedrawing in which the reference number first appears.

FIG. 1 illustrates an example system having multiple transmittersaccording to an embodiment of the present invention.

FIG. 2 illustrates a frame having a preamble portion and a data portionaccording to an embodiment of the present invention.

FIG. 3 illustrates sub-carriers in the frequency domain corresponding tomodulated samples of the preamble portion or the data portion shown inFIG. 2 according to an embodiment of the present invention.

FIG. 4 illustrates the sub-carriers of FIG. 3 multiplied by respectivechannel coefficients according to an embodiment of the presentinvention.

FIG. 5 illustrates two sets of channel coefficients, each of whichcorresponds to a different channel, according to an embodiment of thepresent invention.

FIG. 6 illustrates the example system shown in FIG. 1 according toanother embodiment of the present invention.

FIG. 7 illustrates cyclic shifting of samples of an orthogonal frequencydivision multiplexing (OFDM) symbol according to an embodiment of thepresent invention.

FIG. 8 illustrates a frequency response corresponding to the cyclicshifting operation illustrated in FIG. 7 according to an embodiment ofthe present invention.

FIG. 9 is a graphical representation of diversity in the time domainaccording to an embodiment of the present invention.

FIG. 10 illustrates a symbol delay exceeding an example threshold beyondwhich the legacy receiver shown in FIG. 1 will not receive the datacorrectly, according to an embodiment of the present invention.

FIG. 11 illustrates a flow chart of a method of providing cyclic delaydiversity according to an embodiment of the present invention.

FIG. 12 illustrates a flow chart of a method of setting symbol delayaccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

This specification discloses one or more embodiments that incorporatethe features of this invention. The embodiment(s) described, andreferences in the specification to “one embodiment”, “an embodiment”,“an example embodiment”, etc., indicate that the embodiment(s) describedmay include a particular feature, structure, or characteristic, butevery embodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Furthermore, when a particularfeature, structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to effect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

1.0 Overview

FIG. 1 illustrates an example system 100 having multiple transmittersaccording to an embodiment of the present invention. System 100 includesfirst transmitter 110 a, second transmitter 110 b, a legacy receiver120, and channels 130 a-b. First transmitter 110 a transmits informationvia channel 130 a to legacy receiver 120. Second transmitter 110 btransmits a time-delayed version of the information via channel 130 b tolegacy receiver 120.

Legacy receiver 120 combines the information received via channel 110 aand the time-delayed information received via channel 110 b to providecombined information. Legacy receiver 120 processes the combinedinformation as though it is received from a single channel.

In an embodiment, first transmitter 110 a encodes the information beforetransmitting the information to legacy receiver 120, and secondtransmitter 110 b encodes the time-delayed information beforetransmitting the time-delayed information to legacy receiver 120. Legacyreceiver 120 decodes the information received from first transmitter 110a and the time-delayed information received from second transmitter 110b.

According to another embodiment, legacy receiver 120 decodes theinformation and the time-delayed information before combining theinformation and the time-delayed information to provide the combinedinformation. In another embodiment, legacy receiver 120 combines theinformation and the time-delayed information before decoding theinformation and the time-delayed information.

System 100 can include a memory to store a delay that is based on legacyreceiver 120. For instance, transmitter 110 b may predetermine anappropriate delay to be used with respect to legacy receiver 120.According to an embodiment, transmitter 110 b reads the delay from thememory and transmits the time-delayed information based on the delay.

Transmitter 110 a may include a first antenna, and transmitter 110 b mayinclude a second antenna, as depicted in FIG. 1, though the scope of thepresent invention is not limited in this respect. System 100 may furtherinclude a tapped delay line to delay the information for transmission bytransmitter 110 b.

According to an embodiment, channels 130 a-b represent paths throughwhich information is transmitted by transmitters 110 a-b, respectively,to legacy receiver 120. Channels 130 a-b can be figurative or actualpaths.

FIG. 6 illustrates example system 100 according to another embodiment ofthe present invention. In FIG. 6, transmitters 110 a and 110 b includequadrature amplitude modulators (QAMs) 640 a and 640 b, respectively.FIG. 6 will be described with reference to transmitter 110 a, QAMs 640a, and channel 130 a for simplicity, though persons skilled in theart(s) will recognize that the discussion is similarly applicable totransmitter 110 b, QAMs 640 b, and channel 130 b.

Referring to FIG. 6, QAMs 640 a modulate samples of information based ona clock signal generated by or received by respective transmitter 110 a.In FIG. 6, QAMs 640 a can be any type of modulator and need notnecessarily be QAMs. It will be apparent to persons skilled in theart(s) that samples of information can be modulated using any of avariety of modulators in system 100.

For a given time period (e.g., an orthogonal frequency divisionmultiplexing (OFDM) symbol time) QAMs 640 a simultaneously modulate asample of information, with each of the QAMs 640 a operating at adifferent sub-carrier. QAMs 640 a thereby provide a plurality ofmodulated samples, based on the sample that is processed by each of QAMs640 a. Transmitter 110 a transmits the modulated samples to legacyreceiver 120.

In system 100, information is transmitted in frames. FIG. 2 illustratesa frame 200 according to an embodiment of the present invention. Frame200 includes a preamble portion 210 and a data portion 210. Preambleportion 210 and data portion 220 each include multiple modulated samplesP₀-P_(n) and D₀-D_(n), respectively. The number of modulated samples,n+1, is based on the number of QAMs in transmitter 110 a or 110 b thatare transmitting information to legacy receiver 120. A modulated sampleis transmitted by each QAM of the respective transmitter 110 a or 110 b.The number of modulated samples in preamble portion 210 or data portion220 equals the number of QAMs.

Legacy receiver 120 uses preamble portion 210 to determine/estimate achannel via which frame 200 is received. The channel can be determinedusing a time domain technique, a frequency domain technique, any othersuitable technique that allows for determination or estimation of thechannel, or any combination thereof. According to an embodiment, anerror minimization algorithm is employed to improve the channelestimation error performance of a technique.

Once legacy receiver 120 determines/estimates the channel, legacyreceiver 120 is able to process data portion 220 of frame 200. Thus,legacy receiver 120 processes preamble portion 210 before processingdata portion 220.

2.0 Transmission Via a Single Channel

FIGS. 3 and 4 provide a frequency domain representation of samples, suchas modulated preamble samples P₀-P_(n), of preamble portion 210 ormodulated data samples D₀-D_(n) of data portion 220 shown in FIG. 2according to embodiments of the present invention. FIGS. 3 and 4 aredescribed with respect to a single channel for simplicity. Thisdiscussion is expanded in section 3.0 below to show the frequency domainrelationship between multiple channels.

FIG. 3 illustrates sub-carriers f₀-f_(n) in the frequency domaincorresponding to modulated preamble samples P₀-P_(n) of preamble portion210 or modulated data samples D₀-D_(n) of data portion 220 according toan embodiment of the present invention. In FIG. 3, each sub-carrierf₀-f_(n) carries one respective modulated sample P₀-P_(n) or D₀-D_(n) ofinformation. Each sub-carrier f₀-f_(n) represents a complex number whosevalue depends on the modulated data, based on a standard associated withframe 200. The standard may be a cellular, wireless metropolitan areanetwork (WMAN), wireless local area network (WLAN), wireless personalarea network (WPAN), or wireless fidelity (Wi-Fi) standard, or any otherwireless standard.

The combination of sub-carriers f₀-f_(n) corresponding to modulatedsamples P₀-P_(n) or D₀-D_(n) is referred to as frequency content 300 ofpreamble portion 210 or data portion 220, respectively. Frequencycontent 300 of preamble portion 210 is said to be semi-random, becauseeach standard specifies certain sub-carriers of preamble portion 210 tobe negative to more closely resemble a random pattern. For instance,sub-carriers f₁, and f₂ are shown to be negative in FIG. 3 forillustrative purposes. The inclusion of negative sub-carriers reducesthe number and/or magnitude of peaks in the time domain signalcorresponding to modulated samples P₀-P_(n).

Because preamble portion 210 is used for channel estimation, setting themagnitude of f₀-f_(n) to be plus or minus one helps to reduce thecomplexity of the channel estimator at the receiver. In the embodimentof FIG. 3, legacy receiver 120 determines/estimates the channel viawhich modulated samples P₀-P_(n) or D₀-D_(n) are received based on thecombination of positive and negative ones representing frequency content300 of preamble portion 210.

Transmitters 110 a-b and legacy receiver 120 usually are not optimizedfor the current conditions of a channel, because the channel changeswith time. Some modulated samples P₀-P_(n) or D₀-D_(n) may have agreater magnitude than others. As shown in FIG. 4, each sub-carrierf₀-f_(n), is multiplied by a respective channel coefficient h₀-h_(n)according to an embodiment of the present invention. The magnitudes ofchannel coefficients h₀-h_(n) shown in FIG. 4 are provided forillustrative purposes only, and are not intended to limit the scope ofthe present invention. The magnitudes of channel coefficients h₀-h_(n)depend on characteristics of the channel. According to an embodiment,legacy receiver 120 performs a transform, such as a fast Fouriertransform (FFT) on modulated samples P₀-P_(n) or D₀-D_(n) to providesub-carriers f₀-f_(n) that are multiplied by respective channelcoefficients, as illustrated in FIG. 4.

Because channel coefficients h₀-h_(n) are completely random, some of thechannel coefficients h₀-h_(n) are small (e.g., h₁) as compared to theother channel coefficients h₀-h_(n). When a sub-carrier f₀-f_(n) havinga small channel coefficient h₀-h_(n) is received by legacy receiver 120,the noise in system 100 can interfere and/or prevent reception of thesub-carrier f₀-f_(n). If legacy receiver 120 cannot adequately process asub-carrier f₀-f_(n), an error occurs on the sub-carrier.

Referring to FIG. 4, each sub-carrier f₀-f_(n) has only one channelcoefficient h₀-h_(n) because frame 200 is transmitted via a singlechannel. If the channel is small for a sub-carrier f₀-f_(n), then frame200 may be lost, assuming that error correcting code is not capable ofrecovering frame 200.

3.0 Transmission Via Multiple Channels

FIG. 5 illustrates two sets of channel coefficients h¹ ₀-h¹ _(n) and h²₀-h² _(n), each of which corresponds to a different channel, accordingto an embodiment of the present invention. For example, channelcoefficients h¹ ₀-h¹ _(n) and h² ₀-h² _(n) can correspond to channels130 a and 130 b, respectively, in FIG. 1. Each channel operatesindependently from the other channel(s). Thus, channel coefficients h¹₀-h¹ _(n) are independent of channel coefficients h² ₀-h² ^(n).Corresponding channel coefficients h¹ ₀-h¹ _(n) and h² ₀-h² _(n) arelikely to be different at any given time. If a channel coefficientassociated with one of the channels is relatively small, the channelcoefficient associated with the other channel may be larger. If thesetwo channel coefficients are combined, then the frame is less likely tobe lost. Legacy receiver 120 combines channel coefficients h¹ ₀-h¹ _(n)and h² ₀-h² _(n) to provide combined channel coefficients forprocessing.

If the same information is transmitted via multiple channels of asystem, then lobes are created, such that channel coefficients h¹ ₀-h¹_(n) and/or h² ₀-h² _(n) are equal to zero at points in space thatdepend on the configuration of the system. The system can utilize cyclicdelay diversity to mitigate this effect.

Cyclic delay diversity is achieved by transmitting multiple versions ofthe same information using multiple transmitters, such as transmitters110 a-b in FIG. 1. At least one version of the information is cyclicallyshifted with respect to another version.

Referring to FIG. 7, samples s₀-s_(n) of an orthogonal frequencydivision multiplexing (OFDM) symbol are cyclically shifted according toan embodiment of the present invention. Samples s₀-s_(n) can be samplesP₀-P_(n) of preamble portion 210 or samples D₀-D_(n) of data portion 220of FIG. 2, to provide some examples. Transmitter 110 a transmits a firstversion of the OFDM symbol in accordance with a function x(t).Transmitter 110 b transmits a second version of the OFDM symbol inaccordance with a function x(t+τ). The second version includes the samesamples s₀-s_(n) as the first version, though the second version istime-shifted with respect to the first version. The time shift isrepresented by the variable, τ, and is illustrated in FIG. 7.

Each sample of x(t+τ) is shifted in time by a number of samples L thatcorresponds to the time delay, τ. The time delay can be represented bythe following equation:τ=L·dt  (Equation 1)

where dt is the time necessary for transmitter 110 a or 110 b totransmit one sample.

In the embodiment of FIG. 7, seven samples s₀-s_(n) are transmitted byeach transmitter 110 a-b, and samples s₀-s_(n) transmitted bytransmitter 110 b are shifted by three samples in the time domain withrespect to samples s₀-s_(n) transmitted by transmitter 110 a. Thus, inthe embodiment of FIG. 7, n=6 and L=3. In this embodiment, the timedelay, τ, is equivalent to the time necessary for transmitter 110 a or110 b to transmit three samples.

As shown in FIG. 7, the shifting operation can be performed such thatsamples s₀-s_(n) of the OFDM symbol are rotated in a time frame of theOFDM symbol on a first-in-first-out basis. In other words, as samples ofthe OFDM symbol are shifted beyond the time frame of the OFDM symbol,those samples are re-directed to the beginning of the time frame. As asample is shifted from the end of the time frame to the beginning of thetime frame, all other samples in the time frame are shifted a timeequivalent to dt.

According to an embodiment, the time delay, τ, is programmable. Forinstance, the time delay, τ, can be based on the ability of legacyreceiver 120 to process a signal that includes time shifted samples.

FIG. 8 illustrates a frequency response 800 corresponding to the cyclicshifting operation illustrated in FIG. 7 according to an embodiment ofthe present invention. The cyclic shifting operation can be representedby the following Fourier transform:S _(m−L) →S _(m) ·e ^(−jLmΔf)  (Equation 2)

where m=0, . . . , n. The variable n represents one less than the totalnumber of channel coefficients h₀-h_(n) associated with the cyclicshifting operation. In FIG. 8, the cyclic shifting operation isrepresented by the Fourier transform for illustrative purposes only.Persons skilled in the relevant art(s) will recognize that the cyclicshifting operation may be performed using any of a variety oftransforms.

As shown in Equation 2, a time shift in the time domain translates to aphase shift in the frequency domain. Thus, legacy receiver 120 canprocess samples s₀-s_(n) that have been cyclically shifted by using atime domain technique, a frequency domain technique, or a combinationthereof.

Referring to frequency response 800 in FIG. 8, legacy receiver 120combines corresponding channel coefficients h¹ ₀-h¹ _(n) and h² ₀-h²_(n)e^(−jnLΔf) to provide combined channel coefficients h*₀-h*_(n). Withfurther reference to frame 200 in FIG. 2, legacy receiver 120 combineschannel coefficients corresponding to preamble samples P₀-P_(n) todetermine the channel via which preamble samples P₀-P_(n) are received.However, legacy receiver 120 does not receive preamble samples P₀-P_(n)via a single channel. Instead, legacy receiver 120 receives a firstversion of preamble samples P₀-P_(n) via channel 130 a and atime-shifted version of preamble samples P₀-P_(n) via channel 130 b.Legacy receiver 120 estimates the channel to be a combination ofchannels 130 a-b, based on combined channel coefficients h*₀-h*_(n).Thus, legacy receiver estimates the channel as if the first version ofpreamble samples P₀-P_(n) and the time-shifted version of preamblesamples P₀-P_(n) are received via a single channel.

After estimating the channel based on the combined channel coefficientscorresponding to preamble samples P₀-P_(n), legacy receiver 120 combineschannel coefficients of a first version of data samples D₀-D_(n)received via channel 110 a and a time-shifted version of data samplesD₀-D_(n) received via channel 110 b. Legacy receiver 120 is therebycapable of processing the first version and time-shifted version of datasamples D₀-D_(n) as though they are received via the single channelestimated by legacy receiver 120.

A channel coefficient of a data sample transmitted by transmitter 110 acan be represented as h¹ _(m). As shown in Equation 2, a channelcoefficient of a corresponding time shifted data sample transmitted bytransmitter 110 b can be represented as h² _(m)·e^(−jLmΔf), where Δf isthe phase shift in the frequency domain that corresponds to the timeshift in the time domain. The combined channel coefficient h*_(m) foreach data sample D₀-D_(n) can be represented by the following equation:h* _(m) =h ¹ _(m) +h ² _(m) ·e ^(−jLmΔf)  (Equation 3)

where m represents the data sample D₀-D_(n) with which the combinedchannel coefficient is associated. The mathematical relationship betweendata samples D₀-D_(n) received from transmitters 110 a and 110 b is thesame as the mathematical relationship between preamble samples P₀-P_(n)received from transmitters 110 a and 110 b. Legacy receiver 120 istherefore capable of decoding data samples D₀-D_(n) received fromtransmitters 110 a and 110 b based on the estimated channel.

The composite signal h*·S received by legacy receiver 120 can berepresented as follows:

$\begin{matrix}{{h^{*} \cdot S} = {{\sum\limits_{m = 0}^{n}{h_{m}^{*} \cdot S_{m}}} = {\sum\limits_{m = 0}^{n}{\left\lbrack {h_{m}^{1} + {h_{m}^{2} \cdot e^{{- j}\; L\; m\;\Delta\; f}}} \right\rbrack \cdot S_{m}}}}} & \left( {{Equation}\mspace{14mu} 4} \right)\end{matrix}$

According to an embodiment, if one of h¹ _(m) or h² _(m) is small, theother of h¹ _(m) or h² _(m) is likely to be large enough such thatlegacy receiver 120 can accurately process the combined channelcoefficient h*_(m).

FIG. 9 is a graphical representation of diversity in the time domainaccording to an embodiment of the present invention. FIG. 9 shows threeplots. The first plot, x(t), represents a symbol in the time domain. Thesecond plot, x(t+τ), represents the symbol time-shifted by a period oftime, τ. The third plot, x*(t), represents the combination of thesymbol, x(t), and the time-shifted symbol, x(t+τ).

Referring to FIG. 9, the transmitter 110 a transmits signal x(t) havingpeak A and peak B. Transmitter 110 b transmits signal x(t+τ) having peakA′ and peak B′ corresponding to respective peaks A and B of signal x(t).Legacy receiver 120 combines signals x(t) and x(t+τ) to provide thecombined signal x*(t), which can be represented by the followingequation:x*(t)=x(t)+x(t+τ)  (Equation 5)

In FIG. 9, the combined signal x*(t) includes peaks A, B, A′, and B′.Thus, the combined signal has greater diversity than signal x(t) orsignal x(t+τ) alone.

With respect to frequency domain channel determination/estimationtechniques, the channel estimation capabilities of legacy receiver 120are independent of the symbol delay L. However, legacy receiver 120 maynot be capable of determining/estimating the channel using a time domaintechnique if the symbol delay L exceeds a threshold associated withlegacy receiver 120. To illustrate this point, FIG. 10 shows a symboldelay exceeding an example threshold of legacy receiver 120 according toan embodiment of the present invention.

In the embodiment of FIG. 10, legacy receiver 120 has a threshold of 100ns for illustrative purposes. Thus, legacy receiver 120 can accuratelyestimate the channel using a time domain technique, so long as thechannel does not exceed 100 ns. However, the scope of the presentinvention is not limited to the embodiment of FIG. 10. Differentreceivers can have different capabilities, and legacy receiver 120 canhave any suitable threshold.

In the first plot of FIG. 10, legacy receiver 120 is capable ofaccurately estimating the channel, because the channel does not exceed100 ns. However, in the second plot of FIG. 10, the channel is expandedusing a time domain channel determination technique. Legacy receiver 120is not capable of estimating the expanded channel in the second plotbecause the expanded channel exceeds 100 ns. In the embodiment of FIG.10, the delay employed by the time domain channel determinationtechnique expands the channel beyond the capabilities of the receiver.

According to an embodiment, the symbol delay L that is applied to asignal transmitted by transmitter 110 b is variable. In anotherembodiment, the symbol delay L is determined on a receiver-by-receiverbasis. The symbol delay L can be determined based on an algorithm usinga high layer of transmitter 110 b, such as the MAC layer.

FIG. 11 illustrates a flowchart of a method of providing cyclic delaydiversity according to an embodiment of the present invention. Theinvention, however, is not limited to the description provided by theflowchart 1100. Rather, it will be apparent to persons skilled in therelevant art(s) from the teachings provided herein that other functionalflows are within the scope and spirit of the present invention.

Flowchart 1100 will be described with continued reference to examplesystem 100 described above in reference to FIG. 1. The invention,however, is not limited to this embodiment.

Referring now to FIG. 11, transmitter 110 a transmits first data viafirst channel 130 a at block 1110. Transmitter 110 b transmitstime-shifted first data via second channel 130 b at block 1120. Legacyreceiver 120 processes the first data and the time-shifted first data atblock 1130 as though the first data and the time-shifted first data arereceived via a single channel. First data can be a frame, a data portionof a frame, a preamble portion of a frame, a symbol (e.g., an orthogonalfrequency division multiplexing symbol), or any other suitable groupingof information.

According to an embodiment, a time delay associated with thetime-shifted first data is programmable. For example, transmitter 110 bcan determine or select the time delay based on a model number or apredetermined threshold associated with legacy receiver 120. In anotherexample, transmitter 110 b iteratively sets the time delay until legacyreceiver 120 indicates proper receipt of information having the timedelay.

In an embodiment, legacy receiver 120 determines the single channelbased on second data received via channel 110 a and second time-shifteddata received via channel 110 b. For example, the first data can be adata portion of an orthogonal frequency division multiplexing (OFDM)frame, and the second data can be a preamble portion of the OFDM frame.

FIG. 12 illustrates a flowchart of a method of setting symbol delayaccording to an embodiment of the present invention. The invention,however, is not limited to the description provided by the flowchart1200.

Rather, it will be apparent to persons skilled in the relevant art(s)from the teachings provided herein that other functional flows arewithin the scope and spirit of the present invention.

Flowchart 1200 will be described with continued reference to examplesystem 100 described above in reference to FIG. 1. The invention,however, is not limited to this embodiment.

Referring now to FIG. 12, transmitter 110 b transmits a symbol using apredetermined maximum symbol delay, L_(max), at block 1210. If legacyreceiver 120 correctly receives the symbol (i.e., correctlydetermines/estimates the channel via which the symbol is transmitted),as determined at decision block 1220, then the data portion of the framewill also be received without error, in the absence of otherimpairments. Transmitter 110 b is notified of the correct datareception, for example via a positive acknowledgement, and the flowends. If legacy receiver 120 does not correctly receive the symbol(i.e., does not correctly determine/estimate the channel), then the dataportion of the frame is received in error. Transmitter 110 b is notifiedof the error, for example via the absence of a positive acknowledgement,and reduces the symbol delay at block 1230. Transmitter 110 bre-transmits the symbol using the reduced symbol delay at block 1240.Control returns to decision block 1220, in which a determination is madeas to whether legacy receiver 120 correctly receives the symbol, whichin the absence of other impairments is equivalent to estimating thechannel correctly. Transmitter 110 b continues to reduce the symboldelay at block 1230 and re-transmit the symbol using the reduced symboldelay at block 1240 until legacy receiver 120 correctly receives thesymbol, as determined at decision block 1220. According to anembodiment, transmitter 110 b may reduce the symbol delay at a MAC layerof transmitter 110 b. For example, the MAC layer may control the numberof re-transmissions.

Utilizing cyclic delay diversity provides many benefits, as compared toconventional wireless systems. For example, cyclic delay diversity canreduce or eliminate the cancellation of energy (i.e., nulls) oftenencountered when transmitting the same data from multiple transmitters.

4.0 Other Embodiments

FIGS. 1-12 are conceptual illustrations allowing an easy explanation ofcyclic delay diversity. These figures and corresponding exemplaryembodiments are described above with reference to a system that includestwo transmitters for illustrative purposes and are not intended to limitthe scope of the present invention. It will be recognized by personsskilled in the relevant art(s) that the techniques described herein areapplicable to systems having any number of transmitters (e.g., three,four, five, etc.). For instance, a receiver may be configured to processdata received via any number of channels from respective transmitters asthough the data is received via a single channel.

It will be further recognized that embodiments of the present inventionmay be implemented using hardware, firmware, software, or anycombination thereof. In such an embodiment, the various components andsteps are implemented using hardware, firmware, and/or software toperform the functions of the present invention. That is, the same pieceof hardware, firmware, or module of software could perform one or moreof the illustrated blocks (i.e., components or steps).

In this document, the terms “computer program medium” and “computerusable medium” are used to generally refer to media such as a removablestorage unit, a hard disk installed in hard disk drive, and signals(i.e., electronic, electromagnetic, optical, or other types of signalscapable of being received by a communications interface). These computerprogram products are means for providing software to a computer system.The invention, in an embodiment, is directed to such computer programproducts.

In an embodiment where aspects of the present invention are implementedin software, the software may be stored in a computer program productand loaded into computer system using a removable storage drive, harddrive, or communications interface. The control logic (software), whenexecuted by a processor, causes the processor to perform the functionsof the invention as described herein.

In another embodiment, aspects of the present invention are implementedprimarily in hardware using, for example, hardware components such asapplication specific integrated circuits (ASICs). Implementation of thehardware state machine so as to perform the functions described hereinwill be apparent to one skilled in the relevant art(s).

In yet another embodiment, the invention is implemented using acombination of both hardware and software.

Any of a variety of transmit modes may benefit from cyclic delaydiversity, including those that need to use only one transmit chain.Some exemplary transmit modes include, but are not limited to, anInstitute of Electrical and Electronics Engineers (IEEE) 802.11astandard, an IEEE 802.11b standard, an IEEE 802.11g standard, and anIEEE 802.11n standard.

5.0 Conclusion

Example embodiments of the methods, systems, and components of thepresent invention have been described herein. As noted elsewhere, theseexample embodiments have been described for illustrative purposes only,and are not limiting. Other embodiments are possible and are covered bythe invention. Such other embodiments will be apparent to personsskilled in the relevant art(s) based on the teachings contained herein.Thus, the breadth and scope of the present invention should not belimited by any of the above described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

1. A system, comprising: a first transmitter to transmit data via afirst channel; and a second transmitter to transmit, time-shifted, thedata via a second channel; wherein the second transmitter is configuredto repeatedly transmit the time-shifted data, each time with a delaythat is reduced with respect to a previous transmission, until receivingan indication of proper receipt.
 2. The system of claim 1, wherein thefirst transmitter transmits the data and the second transmittertransmits the time-shifted data in accordance with at least one of anInstitute of Electrical and Electronics Engineers (IEEE) 802.11astandard, an IEEE 802.11b standard, an IEEE 802.11g standard, and anIEEE 802.11n standard.
 3. The system of claim 1, wherein a time delayassociated with the time-shifted data is programmable.
 4. The system ofclaim 1, further comprising a memory to store the delay associated withthe time-shifted data.
 5. A method, comprising: transmitting data via afirst channel; transmitting, time-shifted, the data via a secondchannel; and receiving a determination of proper receipt of the data; ifthe data was not properly received, retransmitting the time-shifted datawith a reduced delay between retransmitting via the first and secondchannels.
 6. The method of claim 5, wherein transmitting the data andtransmitting the time-shifted data are performed in accordance with atleast one of an Institute of Electrical and Electronics Engineers (IEEE)802.11a standard, an IEEE 802.11b standard, and an IEEE 802.11gstandard.
 7. The method of claim 5, wherein a time delay associated withthe time-shifted data is programmable.
 8. The method of claim 5, furthercomprising storing the delay associated with the time-shifted data.
 9. Asystem, comprising: means for providing data via a first channel; meansfor providing, time-shifted, the data via a second channel, wherein themeans for providing the time-shifted data repeatedly transmits thetime-shifted data, each time with a delay that is reduced with respectto a previous transmission, until receiving an indication of properreceipt.
 10. The system of claim 9, wherein the means for providing thetime-shifted data includes an antenna.
 11. The system of claim 9,wherein the means for providing the time-shifted data includes anequalizer.
 12. The system of claim 9, wherein the means for providingthe time-shifted data includes at least one of software and firmware.